Swim fins are generally known and typically include a closed toe foot pocket and a blade portion. A desirable feature of a swim fin is that the blade portion of the fin easily attains a correct “angle of attack”. The angle of attack is the relative angle that exists between the oncoming flow (i.e., direction of motion of the user) and the actual lengthwise alignment of the blade of the fin. A “correct angle of attack” optimizes the conversion of kicking energy of the user to thrust or propulsion through the water (and in the case of a tail fin maximizes the lift generated by the hydrofoil shape of the tail fin). When this angle is small, the blade is at a low angle of attack. When this angle is high, the blade is at a high angle of attack. As the angle of attack increases, the flow collides with the fins attacking surface at a greater angle. This increases fluid pressure against this surface for the blade (but decreases the surface pressure for the tail fin as it is creating lift). The propulsion is achieved either through drag propulsion (creating a void with the blade and being pulled into that void) or through lift (creating a lower pressure through the Bernoulli principle like an airplane wing). When using lift propulsion, the ability to increase the frequency of the sinusoidal wave created by the kicking stroke while decreasing the amplitude (the distance between the fins when they are at their farthest distance apart) to generate higher thrust with reduced drag is desirable enhancement to swim fin performance.
Current and traditional fins tend to assume different curvatures to form their attack angles according to the direction of movement and the magnitude of the forces applied during use (i.e., the amount of energy or power in the kick and the amplitude of the kicking stroke). Designing a swim fin to provide a particular angle of attack for a particular amount of power is generally known. One way to design a fin for a particular kicking power is to alter the composition of the material (e.g., stiff material for hard kicking, flexible or soft material for light kicking, etc.). Changing the composition of the material, however, does not efficiently or adequately control the angle of attack because of the unknowns manifested in compliant geometry. Most existing fins can only reach a compromise in that they are either stiff, soft, or somewhere in between. When conventional fins are designed for hard kicking (e.g., made of stiff material), they reach the correct angle of attack when kicked very hard. On a normal, relaxed kick they don't bend far enough and this negatively affects the performance. Fins of this kind will be uncomfortable on the legs, strenuous and with poor performance on a relaxed dive. When conventional fins are designed for light kicking (e.g., made of soft material or made with large vents or splits), they reach the correct angle of attack when kicked very gently. With a strong kick, such as when swimming in a current or needing to get up to speed, the blade is overpowered and there is little or no thrust available because a small void is created poorly. Fins like this might be comfortable on a relaxed dive, but could become unsafe by not being able to provide the thrust to overcome a slight current. When conventional fins are somewhere in between, they can be overpowered when kicked real hard, are still uncomfortable when kicked gently, but cover a wider range of useful kicking power.
When such known fins are used outside their prescribed kicking power, the angle of attach tends to be too low or too high. When the fin blade is at excessively high or low angles of attack, the flow begins to separate, or detach itself from the low pressure surface of the fin. This tends to cause the fin to be less efficient. Another problem that occurs at higher angles of attack is the formation of vortices along the outer side edges of the fin. This tends to cause unwanted drag. Drag becomes greater as the angle of attack is increased. This reduces the ability of the user to create a significant difference in pressure (by creating a void) between its opposing surfaces for a given angle of attack, and therefore decreases the power delivered by the fin.
Most swim fins have reinforcing ribs for the blade to help give the generally flat flexible material of the blade enough structural support so as to give an appropriate amount of flex for the blade. These ribs also serve to keep the blade from causing the closed toe foot pocket from collapsing in a crease that crushes the toes of the user as is explained in more detail later. Some blades have splits to allow the water to flow through with less resistance and some are longer and some are shorter. Some blades are foil shaped to increase the laminar flow over the surfaces, but most are simply flat planes with supporting ribs. The large majority of fins historically produced and in use at present are the closed toe embodiment of foot pockets.
In the present state of the are, the blade for of each fin starts its bending several inches in front of the toes of the foot pocket. McCarthy's U.S. Pat. No. 6,884,134 has an extensive description of the prior art as of its 2003 filing. In this overview of the art, it is clear that the closed toed foot pockets presented there, describing a broad review of the art, consistently have blades whereby the blades bend several inches in front of the toe section of the foot pocket. This increases the effort needed to use these fins in comparison to the same fin that would bend to the proper angle of attack in close proximity to the ball of the user's foot. Any work done further from the heel requires more energy because of centrifugal forces. This principle is disclosed and better explained in Melius U.S. Pat. No. 6,893,307.
Other swim fins may have vents or apertures in front of to the toe section of the foot pocket. These vents or apertures have been designed to relieve some of the water pressure on that part of the blade and possibly to enhance water flow over the blade. The vents or apertures do not free the toe section from the plane of the blade so that it can more independently from the plane of the blade. Thus, the blade works to stiffen the toe section so that it will not crease towards the toes of the user as is disclosed later in this patent. These swim fins are difficult to bend near the foot pocket because the closed toes foot pocket generally has the shape of a truncated irregular cone to help seat the foot. This truncated irregular cone shape for the foot pocket is very difficult to bend or deform even with the use of soft flexible materials because this type of geometric shell acts something like an arch. It doesn't bend evenly, but rather breaks at in a crease causing undue pressure on the toes of the user. Thus, the vast majority of swim fins are stiffened on either side of the foot pocket so that the blade will flex on an axis several inches down the blade away from the foot pocket.
It is also apparent that open toed foot pockets bend further down the blade from where the toes protrude from the foot pocket. In some open toed variations of foot pockets for swim fins such as those disclosed in Melius' U.S. Pat. Nos. 6,893,307 and 7,083,485, the blade has an axis of flexing somewhat closer to the toes as is disclosed in more detail later in this patent. In this case, the intersection of the foot pocket with the blade still needs a certain amount of increased stiffness because it can develop material failures at the open toe to blade intersection. Because the material finds an edge at this intersection, stress on this edge can start rips in the material. The swim fins found in Evans' U.S. Pat. Nos. such as 6,354,894; 5,417,599; and 4,857,024 all have blades with open toed foot pockets, but the blades are designed and functionally bend in front of the toes of the users to relieve the stresses that would otherwise rip the material at the intersection of the foot pocket and the blade. The blade open toed foot pocket interface has to be stiff to withstand the forces of flexing during normal use at that intersection, and this limits the flexibility of the blade near this intersection.
Thus, it would be advantageous to provide a swim fin that provides a desired or optimum angle of attack for a range of kicking strengths and a variety of amplitudes (the distance that the fins travel from one extreme to the other during one cycle in kicking) in the kicking stroke. It would further be desirable to provide a swim fin in which the angle of attack is accurately controlled both for the upstroke and for the down stroke so that the ratio of power to fin area is markedly increased (which makes it possible to reduce the overall size of the swim fin without sacrificing total power) for various kicking efforts. It would further be advantageous to be able to change a small portion of the fin to better be able to adjust the performance characteristics of the fin through empirical testing thus allowing the altering the mold with a relatively inexpensive insert for the mold in the manufacturing process to create a larger or smaller relief jet aperture to alter the fin for various types of kicking strengths and energies. This would be advantageous by controlling the angle of attack by controlling the structural characteristics of the bending of the blade and not by altering the characteristics of materials which would enhance the empirical control of the bending of the blade. It would further be desirable to provide a swim fin with living hinges that increase the performance by controlling the angle of attack and converting a higher percentage of the kick energy into thrust while reducing the energy needed to deform the blade into the proper angle of attack because the blade bends in close proximity to the ball of the user's foot. It would further be advantageous to provide a swim fin with flow characteristics that pull the water into the center of the blade (and tail fin when a tail fin is used) and provides improved water flow characteristics by reducing drag through the generation of side vortices. It would further be desirable to have a swim fin that increased speed and thrust with an increase in smaller kicking stoke amplitudes while increasing the frequency of the stroke. It would further be desirable to provide for a swim fin having one or more of these or other advantageous features.
To provide an inexpensive, reliable, and widely adaptable swim fin with improved angle of attack (for both non-lift-generating surfaces and lift-generating surfaces such as foil shaped blades and tail fins), improved efficiency with the relief jet aperture aperture allowing the blade to bend in close proximity to the ball of the user's foot, improved methods for swimming with lower drag kicking techniques representing a significant advance in the art.